This invention is directed to imaging techniques. More particularly, embodiments the invention provide a method and system for capturing images on CMOS color sensors. Merely by way of example, embodiments of the invention have been used to reduce and eliminate blooming effects in image capturing. But it would be recognized that the invention has a much broader range of applicability.
To capture vivid imageries have long been an endeavor that persisted as long as the human race itself. As early as the Stone Age, people tried to capture of what they see with cave drawings. Over the thousands of years, artists developed techniques for capturing images with paint brushes and canvases. With oil paintings, artists have been able to capture real world images with accuracy, but the fidelity of paintings is no match to photography.
In 1826, a French inventor Nicéphore Niépce produced the first photographic image on polished pewter plate covered with a petroleum derivative. Since then, the technique for photographic imaging evolved. Better techniques and equipment improved image quality over the next hundred of years. Over the last fifty years, techniques for color photography have been improved and consummated. In the last decade, with the introduction of the first commercially available digital camera by Kodak in 1990, a new type of image capturing technique, digital imaging, has rapidly become a popular way for capturing images.
For digital imaging, the image sensor (or digital equivalent of film negatives) is one of the most important components for digital imaging devices, such as digital cameras, camera phones, etc. For a long time, image sensors have been based on charge-coupled device (CCD) technology that has been developed by George Smith and Willard Boyle at Bell Labs. In the past, CCD based imaging devices dominated. Recently, CMOS based image sensors have become more popular.
The CMOS image sensor technology typically includes millions of sensor pixels (light sensing units), each of the sensor pixel includes two to four transistors and a photodiode. Typically, conventional techniques for CMOS image sensing use one np junction, with very shallow p+ layer applied on top of N region to reduce noise and enhance blue response in image capturing process. In a way, the CMOS sensor unit works in a similar manner as a capacitor. The more charge stored in the electrode, the higher the voltage drop across the depletion region of the CMOS. Light, which is an energy source, generates free carriers. The free carriers under electric field run towards the N type region of the CMOS sensor and neutralized the charge and reduce potential. The voltage difference before and after the integration of energy provides a signal level. The signal level is then used as a reading as the amount of light being detected and used for forming an image.
Depending upon applications, CMOS sensor often have advantages over CCD sensors. For example, compared to CCD image sensors, CMOS sensors usually provide lower costs and longer battery life. As a result, CMOS is often preferred for portable imaging devices such as camera phone and point and shoot cameras. At the high end, CCD image sensors are often behind CMOS images sensors in terms of noise level and sensitivity. Because of various advantages of CMOS image sensors, the technologies for CMOS image sensors have been rapidly developing.
The capability of image sensor is often measure by the number of image sensors (or pixels). In the recent years, camera makers have been racing one another for producing new cameras with higher pixel count. The resolution of CMOS image sensor has been increasing as pixel size shrink together with the MOS transistor channel length. While the increase resolution of image sensors often improve image resolution, the decreased pixel size and increase noise level have become obstacles for improved image quality. As a result of higher pixel density and decreased pixel size, images produced for CMOS censors often suffer from unwanted artifacts such as moir, fringing, blooming, etc.
Various conventional techniques have been developed to improve image quality and to reduce and even eliminate unwanted artifacts. However, conventional techniques are often inadequate.
Therefore, it is desirable to have an improved method and system for a CMOS image sensing device.